Technical Field
This disclosure relates to communication systems supporting simultaneous transmit and receive, co-existent communication devices, systems requiring enhanced isolation between certain ports or blocks, and other systems where signal cancellations are achieved through quadrature hybrids.
Description of Related Art
In a Frequency Division Duplex (FDD) radio communication system, the transmitter (TX) and the receiver (RX) of the same radio may operate simultaneously, but, in two different frequency bands, ftx for TX and frx for RX. In such scenarios, an important performance metric for the radio communication system may be the degree of “isolation” between the TX and the RX that are operating simultaneously. The isolation can be particularly important within the TX frequency band and within the RX frequency band. Any leakage that reaches the RX within the TX frequency band or the RX frequency band may interfere significantly with the receiver.
On the other hand, many platforms include several communication devices at close proximity. Examples may include commercial or military platforms, such as handheld or portable platforms, that include various voice and data communication transceivers as well as wireless positioning solutions. In these platforms, various communication devices may operate at the same time, resulting in unwanted interference among them. In such co-existence scenarios, high isolation between various simultaneously-operating communication devices may be needed to ensure proper operation.
The minimum required isolation may depend on the application and the scenario. For example, in a typical commercial FDD radio, TX to RX isolation of 50 dB or more may be required in the TX and the RX frequency bands. Without adequate isolation, the aggressor TX signal may significantly deteriorate the sensitivity of the victim RX and ultimately prevent its proper operation. In a co-existence radio, the isolation between the platforms may need to be even higher.
One approach to providing TX-RX isolation is to use a frequency duplexer. The frequency duplexer is a three port electrical network. One port is typically connected to the antenna (ANT), one port is typically connected to the TX output, and one port is typically connected to the RX input.
A common challenge in duplexer design is to achieve low insertion loss from TX to ANT and from ANT to RX, while providing high isolation from TX to RX. However, meeting this requirement may require costly resonator technologies, such as Bulk Acoustic Wave (BAW) resonators.
Another approach to enhancing isolation between TX and RX is to generate a cancellation signal that fully or partially matches the amplitude of the leakage signal from the aggressor TX to the victim RX, but with the opposite phase (negative sign). The cancellation signal is then combined with the leakage signal to cancel it out and enhance isolation.
One example of a cancellation network utilizes quadrature hybrids and other components to ensure an acceptable amount of isolation among various ports.
A variety of non-idealities, such as component mismatches, path imbalances, finite component isolations, impedance mismatches, and varying antenna mismatch, however, can diminish the effectiveness of the cancellation and thus the degree of isolation that is achieved, as well as the amount of signal return losses at an impedance mismatched port.